In U.S. Pat. No. 2,987,465 to Johnson, there is disclosed a reaction system between liquids, solids and gases in a so-called "ebullated bed" in a reaction vessel in which gases and liquids flow upwardly through a reaction zone under conditions which tend to place the contact particles in random motion. It has been found that nearly isothermal conditions result when the catalyst or contact bed is expanded at least 10%, and generally not more than 300%. It is also found that the random motion of the particles in the liquid avoids attrition and permits such control that substantially no solids are carried out of the reaction zone. In each ebullated system, variables which may be adjusted to attain the desired ebullation (random motion of the particles in the liquid) include the flow rate, density and viscosity of the liquid and the gasiform material, and the size, shape and density of the particulate material.
The ebullated bed effect is accomplished by a distributor grid extending laterally within the reaction vessel provided with a plurality of distribution devices, such as bubble caps, and positioned within the reaction vessel by a support ring internally welded to the interior surface of the reaction vessel. With the materials of construction, particularly of the interior surface of the reaction vessel, e.g. low alloy carbon steel covered by stainless steel, weld failure often occurred at the weld or beneath the weld, which failure was difficult to detect and resulted in reduced reaction efficacy.